Astrophysics > High Energy Astrophysical Phenomena

Title:The Origin of Galactic Cosmic Rays

Abstract: One century ago Viktor Hess carried out several balloon flights that led him
to conclude that the penetrating radiation responsible for the discharge of
electroscopes was of extraterrestrial origin. One century from the discovery of
this phenomenon seems to be a good time to stop and think about what we have
understood about Cosmic Rays. The aim of this review is to illustrate the ideas
that have been and are being explored in order to account for the observable
quantities related to cosmic rays and to summarize the numerous new pieces of
observation that are becoming available. In fact, despite the possible
impression that development in this field is somewhat slow, the rate of new
discoveries in the last decade or so has been impressive, and mainly driven by
beautiful pieces of observation. At the same time scientists in this field have
been able to propose new, fascinating ways to investigate particle acceleration
inside the sources, making use of multifrequency observations that range from
the radio, to the optical, to X-rays and gamma rays. These ideas can now be
confronted with data. I will mostly focus on supernova remnants as the most
plausible sources of Galactic cosmic rays, and I will review the main aspects
of the modern theory of diffusive particle acceleration at supernova remnant
shocks, with special attention for the dynamical reaction of accelerated
particles on the shock and the phenomenon of magnetic field amplification at
the shock. Cosmic ray escape from the sources is discussed as a necessary step
to determine the spectrum of cosmic rays at the Earth. In the end of this
review I will also discuss the phenomenon of cosmic ray acceleration at shocks
propagating in partially ionized media and the implications of this phenomenon
in terms of width of the Balmer line emission.